The mark space ratio is a measure of the proportion of the time the output is high.
Back in the days of the Morse code telegraph (wild west movies), the codes were recorded on ticker tape as dots and dashes so there were actual ink marks on the paper tape with spaces between the marks.
Example: If the output was high 80% of the time and low for 20% of the time, the mark space ratio would be 8:2 or 4:1 or just 4.
Mark Space Ratio = tH / tL = high time / low time
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